As a technique of measuring a thermal conductivity of a gas, driving a heater at a fixed temperature in a state where an ambient gas surrounding the heater is maintained at the fixed temperature and measuring a calorific value of the heater is known. This technique utilizes the fact that a calorific value of the heater is in proportion to a thermal conductivity of the ambient gas. However, when adopting this technique to measure a thermal conductivity of the ambient gas, this technique cannot help having a large-scale structure because, e.g., a constant-temperature bath that maintains the ambient temperature at a fixed temperature is required.
Further, Japanese Patent Application Laid-open No. 2001-221758 discloses that providing a cavity that have a fluid accumulated therein so as to face a flow path along which the fluid is led and also providing a porous body at a boundary of this cavity and the flow path enables accurately detecting a thermal conductivity of the fluid from a calorific value of a sensor (a heater) assembled in the cavity. It is to be noted that the porous body is designed so that the fluid in the flow path is exchanged with the fluid in the cavity by molecular diffusion alone.
However, since a thermal conductivity of a gas generally has intrinsic temperature change characteristics according to a type of this gas, there is a fundamental problem that the thermal conductivity cannot be accurately measured by simply measuring a calorific value of a heater. In particular, when a mixed gas in which a plurality of types of gases are mixed like a natural gas is an ambient gas, measuring its thermal conductivity is very difficult.
It is to be noted that there is also adopted a technique of passing a mixed gas to a member called a column, utilizing a difference in flow velocity due to a difference in molecular weight of gases to analyze composition ratios, and measuring a thermal conductivity of the mixed gas. However, such a technique has a problem that analysis of the composition ratios of the mixed gas by using the column requires a long time and an entire structure of an analysis apparatus is completed and expensive, for example.